The invention relates to elements faced with super hard material, and particularly to preform elements comprising a facing table of super hard material having a front face, a peripheral surface, and a rear surface bonded to a substrate of material which is less hard than the super hard material. Preform elements of this kind are often used as cutting elements on rotary drag-type drill bits and the present invention will be particularly described in relation to such use. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform elements for other purposes. For example, elements faced with super hard material, of the kind referred to, may also be employed in workpiece-shaping tools, high pressure nozzles, wire-drawing dies, bearings and other parts subject to sliding wear, as well as elements subject to percussive loads as may be the case in tappets, cams, cam followers, and similar devices in which a surface of high wear resistance is required.
Preform elements used as cutting elements in rotary drill bits usually have a facing table of polycrystalline diamond, although other super hard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail. Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carder, for example by a process known as "L bonding". Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a socket in the bit body, without being brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be molded using a powder metallurgy process. Such cutting elements are subjected to extremes of temperature during formation and mounting on the bit body, and are also subjected to high temperatures and heavy loads when the drill is in use down a borehole. It is found that as a result of such conditions spalling and delamination of the super hard facing table can occur, that is to say the separation and loss of the diamond or other super hard material over the cutting surface of the table. This may also occur in preform elements used for other purposes, and particularly where the elements are subjected to repetitive percussive loads, as in tappets and cam mechanisms.
Commonly, in preform elements of the above type the interface between the super hard table and the substrate has usually been flat and planar. However, particularly in cutting elements for drill bits, attempts have been made to improve the bond between the super hard facing table and the substrate by configuring the rear face of the facing table so as to provide a degree of mechanical interlocking between the facing table and substrate.
One such arrangement is shown in U.S. Pat. No. 5120327 where the rear surface of the facing table is integrally formed with a plurality of identical spaced apart parallel ridges of constant depth. The facing table also includes a peripheral ring of greater thickness, the extremities of the parallel ridges intersecting the surrounding ring.
An alternative arrangement is shown in our co-pending British Patent Application No. 9323207.2 where the rear surface of the facing table is integrally formed with a plurality of circumferentially spaced generally radial ribs, the outer extremities of which intersect a peripheral ring extending rearwardly from the rear surface of the facing table.
In such arrangements the peripheral ring is substantially rectangular in cross-section, although the corners may be rounded. Consequently, the inner surface of the peripheral ring extends substantially at 90.degree. to the rear surface of the facing table so as to be generally parallel to, and face towards, the central axis of the cutting element. As a consequence, the inner surface of the peripheral ring meets the rear surface of the facing table at a substantially 90.degree. angle. It is found in practice that such arrangements may result in two serious disadvantages. Firstly, as is well known, the preform element is formed in a high pressure, high temperature press in a process where the substrate is a preformed solid element having a front surface which is pre-shaped to the required configuration. A layer of diamond particles is then packed on to the configured surface of the substrate, filling the recesses therein and forming a continuous facing layer. Pressing of the combined body in the high pressure, high temperature press causes the diamond particles to be bonded together, with diamond-to-diamond bonding, and also bonded to the surface of the substrate, which is usually cemented tungsten carbide. In order to form the rectangular-sectioned peripheral ring on the rear surface of the diamond layer, the substrate is formed with a corresponding rectangular-sectioned peripheral rebate into which the diamond particles are packed.
It is believed that, due to the rectangular shape of the rebate in the substrate, the diamond particles may be less closely packed in the region of the comer of the rebate and less firmly compressed against the cylindrical inner wall of the rebate, resulting in imperfect bonding between the diamond particles and the material of the substrate in this area. Secondly, the 90.degree. junction between the peripheral ring and the rear surface of the facing table forms a stress concentration at this junction. Both of these features, it is believed, can increase the tendency for the facing table to separate from the substrate in use of the cutting element, when it is subjected to substantial temperatures and stresses. It is an object of the invention to provide a new and improved configuration of cutting element where these disadvantages may be overcome.